The present invention relates to a synchronization control circuit which is effective in simultaneously displaying a plurality of screens in a television set and, more specifically, to a synchronization control circuit for smoothly switching between sync signals to prevent an image from being distorted when the image is switched to another.
Nowadays a number of television (TV) broadcasts such as terrestrial broadcasting, satellite broadcasting and satellite communications broadcasting are provided; accordingly, the number of channels will be increased more and more in the future. TV receivers capable of displaying a plurality of screens at the same time have already appeared, and receivers having a function of simultaneously reproducing graphics images other than broadcasts are increasing in number.
In such a receiver capable of displaying a plurality of screens at once, horizontal and vertical sync signals for an output screen to be displayed on a display are generated based on the sync signal of an image of one of input sources. However, the receiver has a drawback in which a sync signal or image is distorted when the sync signal is switched to another in accordance with channel switching.
In particular, as a device for selectively displaying asynchronous input video signals A1 and B1, there is a display device having its own sync signal generator for generating a sync signal irrespective of sync signals of both the input video signals A1 and B1. The display device necessitates a period of time during which the phase of an internal sync signal (control signal for display) generated by the sync signal generator is synchronized with that of the sync signal of a selected input video signal. In the display device, when one selective state of the input video signal is switched to another, the phase of a vertical sync signal is suddenly changed between the sync signal of the input video signal and the internal sync signal; therefore, a time corresponding to several fields to several tens of fields is required to lock the sync signal, and the sync signal (image) is distorted.
Therefore, a technique of blacking out an image until synchronization is stabilized to prevent it from being displayed, is usually adopted.
In a TV receiver simply for full-screen display, such a blackout is not a serious problem. However, in a TV receiver having a function of simultaneously displaying graphics images which are not involved in channel switching and a received image, the above distortion is a problem.
In a receiver for displaying a graphics image along with an image of an input video signal, if the images are blacked out in accordance with switching between sync signals, a great problem occurs in which auxiliary information of the graphics image disappears during a period of time corresponding to several fields to several tens of fields.
There is a personal computer as an example of a device for displaying graphics. In the computer, all sync signals are generated by means of a system clock of the body thereof and, even though a channel is switched while a moving image is being displayed, its surrounding graphics is not distorted; however, the moving image itself is distorted because the synchronization of a processing system is switched. Since, furthermore, the image is always displayed by asynchronous output signals, a hindrance such as a jump and a continuous display of the image is caused by a difference in synchronization.
On the other hand, a TV receiver mainly aims at displaying a moving image and thus needs to avoid the above hindrance as much as possible.
As described above, in the TV receiver for displaying images from a plurality of sources on a single screen, when a source of a sync signal for display is switched, an image is blacked out and a sync signal (image) is distorted for a while.